Copper-based paste containing copper aluminate for microstructural and shrinkage control of copper-filled vias

ABSTRACT

A copper-based paste is disclosed for filling vias in, and forming conductive surface patterns on, ceramic substrate packages for semiconductor chip devices. The paste contains copper aluminate powder in proper particle size and weight proportion to achieve grain size and shrinkage control of the via and thick film copper produced by sintering. The shrinkage of the copper material during sintering is closely matched to that of the ceramic substrate.

This application is a continuation of U.S. patent application Ser. No.07/758,991 entitled "COPPER-BASED PASTE CONTAINING COPPER ALUMINATE FORMICROSTRUCTURAL AND SHRINKAGE CONTROL OF COPPER-FILLED VIAS", filed Sep.10, 1991, and now issued as U.S. Pat. No. 5,925,443.

BACKGROUND OF THE INVENTION

The present invention generally relates to copper-filled vias in ceramicsubstrates and, more particularly, to a copper-based paste containingcopper aluminate powder in proper particle size and weight proportionfor grain size and shrinkage control of the via and thick film copperproduced by sintering.

The use of copper-filled vias in ceramic substrates and sinteringprocesses for producing them are well known in the semiconductorpackaging art as taught, for example, in U.S. Pat. No. 4,234,367, issuedon Nov. 18, 1980 to Lester W. Herron et al. and assigned to the presentassignee, the disclosure of which is incorporated by reference herein.Recently, more interest has been focused on the associated problems ofthe disparity in shrinkage rates between copper and ceramic as well asthe onset of via "opens", particularly as via diameters are reducedbelow 100 μm in high circuit density applications. A discussion of suchproblems is given in U.S. Pat. No. 4,776,978, issued on Oct. 11, 1988 toLester W. Herron et al. and assigned to the present assignee, thedisclosure of which is incorporated by reference herein.

As set forth in the cited U.S. Pat. No. 4,776,978, metal particles, suchas copper, in the via paste undergo sintering with attendant shrinkageof the thick film pattern (also consisting of the paste) during theinitial phase of the sintering cycle whereas the ceramic and glassparticles (of the ceramic substrate containing the vias) undergosintering during the intermediate and final phases of the sinteringcycle along with their characteristic shrinkage. One method of delayingthe onset of sintering of the metal particles until at least theintermediate phase of the sintering cycle is to intersperse the metalparticles in the thick film with a high melting point material such asaluminum oxide.

Although the foregoing generalized considerations have been known in theart for some time and have provided the basis for techniques forovercoming previous shrinkage and related problems, more refined anddetailed approaches are required to meet the needs of copper-filled viasin ceramic substrates with increasing circuit densities and theconcomitant via diameters in the range of about 85 to 100 μm. It is alsodesirable to provide a copper paste mixture which can be adapted for usewith the next generation of ceramic packages which exhibit reducedshrinkage from sintering.

The following references illustrate previous techniques attempting toovercome shrinkage and other problems.

U.S. Pat. No. 4,594,181, issued on Jun. 10, 1986 to Vincent P. Siuta,teaches the dispersal of copper particles in a solution of anorganometallic compound in an anhydrous volatile organic solvent towardsobtaining a better shrinkage match of copper to ceramic substrate duringsintering.

U.S. Pat. No. 4,599,277, issued on Jul. 8, 1986 to James M. Brownlow etal., discloses the addition of an organometallic compound to a metalmember such as copper paste which compound undergoes decompositionduring sintering to provide a coating such as aluminum oxide on thecopper particles towards obtaining better shrinkage match between copperand ceramic substrate during sintering.

Published European Patent Application, Publication No. 0272129,published Jun. 22, 1988 by Hitoshi Suzuki et al., describes a pastecomposition including a copper powder and an organometallic compoundsuch as an organoaluminate compound, towards obtaining improved adhesionstrength of sintered copper to a ceramic substrate.

U.S. Pat. No. 4,906,405, issued on Mar. 6, 1990 to Seiichi Nakatani etal. and Japanese Patent J63095182, issued on Apr. 26, 1988 to GoeiSeisakusho KK, teach a paste made of copper oxide, and CuAl₂ O₄ as anadditive towards obtaining improved adhesion strength of sintered copperto a ceramic substrate.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a copper paste withappropriate additive to produce copper grain size in the range of about5 to 15 μm after sintering.

Another object is to provide a copper paste with an appropriate additiveto substantially match the shrinkage of the resulting copper material tothe shrinkage, if any, during sintering of a ceramic substrate havingvias filled with said copper paste.

A further object is to provide a copper paste with an appropriateadditive to provide substantially reduced shrinkage after sintering ofthe copper material in low shrinkage porous ceramic substrates havingvias filled with said copper paste.

These and other objects of the present invention, as will be seen from areading of the following specification, are achieved in a preferredembodiment of the present invention by the provision of a copper pastecomprising copper powder, up to about 10 weight percent copper aluminatepowder, and the remainder organic material. Use of copper aluminate inone preferred range from about 0.4 to about 1 weight percent providesthe dual benefits of grain size control as well as shrinkage matchingcontrol of the via copper during sintering.

It is preferred that the size of the copper aluminate particles be inthe range of about 3.0 μm or less because the amount of copper aluminaterequired for grain size control of the sintered Cu varies inversely withcopper aluminate particle size.

In a preferred embodiment of the invention, glass-ceramic particles areadded to the copper-based paste to provide a shrinkage match duringsintering that is substantially identical to that of a glass-ceramicsubstrate.

DETAILED DESCRIPTION OF THE INVENTION

Multilayered glass-ceramic packages for supporting and interconnectingmicroelectronic chip devices can be sintered to a peak temperaturegreater than 950° C. Because of the high temperatures, the chipinterconnecting copper conductors tend to experience exaggerated graingrowth in the vias and in the thick film copper wiring lines.

The growth of large grains in copper is not desirable from the point ofview of reliability. The reason for this is that the plasticity ofcopper varies with the orientation of two neighboring large coppergrains and the grains may separate when they are cooled down from hightemperature and on subsequent thermal cycling. Inasmuch as the conductorsize in both the vias and surface lines is about 70-100 μm, it isdesirable to keep the copper grain size after sintering as small aspossible, namely about 5-15 μm.

In accordance with a first aspect of the present invention, copper grainsize is minimized in a sintering cycles such as the one disclosed in theaforementioned U.S. Pat. No. 4,234,367. Copper grain size is minimizedby adding a small amount of copper aluminate powder to copper powder,mixing with suitable organics to form a paste, and then screening thepaste using a mask on to a green sheet. The green sheet may comprise avariety of materials including, but not limited to, mullite,borosilicate glass, cordierite glass, ceramic, etc. The cordierite glassceramic materials, such as that disclosed in Kumar et al. U.S. Pat. No.4,301,324, the disclosure of which is incorporated by reference herein,are preferred. Preferably, the copper powder has an average particlesize of about 5-8 μm and the copper aluminate powder has an averageparticle size of about 3.0 μm or less.

There are two forms of copper aluminate, namely cupric aluminate (CuAl₂O₄) and cuprous aluminate (CuAlO₂). Unless specifically statedotherwise, whenever copper aluminate is mentioned in this specification,it should be understood that copper aluminate is being used in thegeneric sense to include cupric aluminate and cuprous aluminate, both ofwhich should be considered to be within the scope of the presentinvention.

When suitable conditions are present in a sintering cycle such as taughtin the U.S. Pat. No. 4,234,367, copper aluminate decomposes into copperand alumina according to the following reactions:

    CuAl.sub.2 O.sub.4 +H.sub.2 =Cu+Al.sub.2 O.sub.3 +H.sub.2 O

    2CuAlO.sub.2 +H.sub.2 =2Cu+Al.sub.2 O.sub.3 +H.sub.2 O

The alumina particles produced by the foregoing decomposition reactionsare very small, submicron in size, and are distributed inside the coppermatrix. The presence of a small amount of porosity and the small aluminaparticles inside the copper matrix have been found to inhibit coppergrain growth and result in small copper grains after sintering at hightemperature in excess of 950° C.

More particularly, the unique use of powdered copper aluminate in thecopper paste, in accordance with the present invention, has the specialproperty of yielding grain size control of the sintered copper.

By adding powdered copper aluminate to copper paste, preferably in therange 0.2-1.0% by weight, the sintered copper grain size can be keptsmall. More importantly, the maximum grain size can be kept under about20 μm, which improves the reliability of multilayer ceramic packageshaving copper conductors. It should be noted that by decreasing theparticle size of the copper aluminate powder, smaller copper grain sizescan be obtained with lower weight percentage additions of the copperaluminate to the copper paste.

In general terms, grain size control is the predominant effect when thepowdered copper aluminate paste additive is present up to about 1 weightpercent. Grain size control aids in avoiding opens (breaks) in thesintered copper vias and circuits which have been experienced usingother paste additives which produce much larger copper grain sizes aftersintering.

It has been found that additions to the sintering paste of copperaluminate up to about 10 weight percent are useful for controlling theshrinkage of the sintered copper. With increasing amounts of copperaluminate, but not greater than about 10 weight percent, the sinteredcopper becomes porous, i.e., the copper particles continue to shrinkmicroscopically but not on a macro (global) scale. At about 10 weightpercent copper aluminate, the copper no longer undergoes shrinkage uponsintering. It is more preferred that the copper aluminate be kept atabout 3 weight percent or less since at higher amounts of copperaluminate, the sintered copper has lower strength and increasedelectrical resistivity.

It has further been found that shrinkage control is possible when thereis present, as a minimum, a small but effective amount of copperaluminate. The lower limits have not been determined yet with precision.It is known that about 0.01 weight percent of alumina will induceshrinkage control. It is assumed, therefore, that amounts of copperaluminate (about 0.02 weight percent) that will yield about 0.01 weightpercent alumina will also achieve similar shrinkage control, givensimilar particle sizes. Smaller amounts of copper aluminate are likelyto be effective if the particle size of the copper aluminate, now atabout 3.0 μm, is reduced further.

As is apparent, the effects of grain size control and shrinkage controlmay advantageously overlap at small amounts of copper aluminateadditions.

It would be most desirable to match or substantially match shrinkagecharacteristics during sintering of the copper vias and lines with thatof a glass-ceramic, particularly a cordierite glass-ceramic, material.Thus, in a preferred embodiment of the invention, there is proposed acopper-based sintering paste comprising copper particles, glass-ceramicparticles, copper aluminate, and suitable organic binder materials.Based on volume percent of the inorganic solids, the paste comprisesabout 90 volume percent copper particles, about 5 to 12 volume percentglass-ceramic particles and about 0.3-1.5 volume percent copperaluminate.

It is preferred that the copper particles have a bimodal distribution.Although a unimodal distribution of the copper particles (preferablyhaving an average particle size of 5-8 μm) will also work well. Morepreferably, there should be about 60-90 volume percent copper particleshaving an average particle size of 5 to 6 μm and 0-30 volume percent ofcopper particles having an average particle size of 1.5 to 2.0 μm. Alsopreferably, the copper aluminate particles should have an averageparticle size of 0.7 to 3.0 μm.

It is anticipated that the present invention will have applicability tomany glass-ceramic materials. The preferred glass-ceramic materials,however, are the cordierite glass-ceramics disclosed in the Kumar et al.U.S. Pat. No. 4,301,324. The average particle size of the glass-ceramicparticles should be about 3.5 μm.

The advantages of the present invention will become more apparent afterreferring to the following examples.

EXAMPLES Examples I

A series of samples were prepared comprising copper particles andvarying amounts of copper aluminate in order to determine the efficacyof copper aluminate as a grain size control agent.

Batches of copper powder particles (from Metz Metallurgical and Dupont),having an average particle size of 6 μm, were mixed with copperaluminate (CuAl₂ O₄), having an average particle size of about 2.5 μm,and various paste additives, including ethyl cellulose resin plus asolvent, wetting agent, and flow control agent. Each batch was dried inan oven at about 100° C. and then milled in a rod mill for 1-2 hours.Thereafter, the paste was pressed into pellets at about 5000 psi.Finally, the pellets were sintered in a sintering cycle such as thatdisclosed in the above Herron et al. U.S. Pat. No. 4,234,367.

The pellets were examined for grain size and the results are illustratedin Table I. As can be seen, the grain size is markedly reduced when atleast 0.2 weight percent copper aluminate is present in the paste.

                  TABLE I                                                         ______________________________________                                        Weight % Copper                                                                             Average Grain                                                                            Maximum Grain                                        Aluminate In  Size in Copper                                                                           Size In Copper                                       Paste         (μm)    (μm)                                              ______________________________________                                        0             18         53                                                   0             >20        >100                                                 0.2           13         55                                                   0.4           13         41                                                   0.5           9          18                                                   0.6           8          15                                                   0.8           8          19                                                   1.0           7          14                                                   ______________________________________                                    

The pellets were also examined for densification, noted as percent oftheoretical density, and resistivity. The results are illustrated inTable II. The samples listed in Table II only used the Metz copperpowder particles. As can be seen, there is a steady decline in percenttheoretical density achieved with increasing amounts of copperaluminate, thus illustrating the ability to control the shrinkage of thesintered copper.

                  TABLE II                                                        ______________________________________                                        Weight % Copper % of Theoretical                                                                          Resistivity                                       Aluminate in Paste                                                                            Density     μ-ohm-cm                                       ______________________________________                                        0.3             93                                                            0.4             91          2.2                                               1.0             80          2.9                                               1.2             76          3.1                                               2.0             73                                                            3.0             69                                                            10.0            55                                                            ______________________________________                                    

Examples II

Samples were prepared comprising copper particles, copper aluminateparticles and glass-ceramic particles in order to determine whether itis possible to match the shrinkage characteristics during sintering of acopper paste and a glass-ceramic material.

A batch of copper-based paste was prepared having the followingcomposition, by volume percent of inorganic materials: 10.08 volumepercent of copper (from Metz Metallurgical) having an average particlesize of 1.5 μm, 79.26 volume percent of copper (from Metz Metallurgical)having an average particle size of 6 μm, 9.9 volume percent ofcordierite glass-ceramic particles (average particle size of 3.5 μm) ofthe composition listed in Table III, and 0.76 volume percent of copperaluminate having an average particle size of 0.6 μm. To this mixture wasadded various past additions, including ethyl cellulose resin plus asolvent, wetting agent, and flow control agent. The resulting mixturewas dried in an oven at about 100° C. and then milled in a rod mill for1 to 2 hours. Thereafter, the paste was pressed into pellets at about5000 psi.

                  TABLE III                                                       ______________________________________                                        Glass-ceramic in       Substrate Glass-                                       paste, weight %        Ceramic, weight %                                      ______________________________________                                        55.0           SiO.sub.2                                                                             55.0                                                   21.23          Al.sub.2 O.sub.3                                                                      21.1                                                   20.0           MgO     22.3                                                   1.0            B.sub.2 O.sub.3                                                                       1.3                                                    2.77           P.sub.2 O.sub.5                                                                       0.3                                                    ______________________________________                                    

Next, a batch of glass-ceramic material (average particle size 3.5 μm)representative of glass-ceramic material in substrates was prepared. Thecomposition is also listed in Table III. The glass-ceramic material wasprepared in a conventional way such as that disclosed in the Herron etal. U.S. Pat. No. 4,234,367, and then pressed into pellets.

Both sets of pellets were sintered according to the sintering cycledisclosed in the above Herron et al. patent as modified by Farooq et al.U.S. patent application Ser. No. 07/672,517, filed Mar. 20, 1991, thedisclosure of which is incorporated by reference herein.

Generally speaking, the sintering cycle proceeds as follows. Thetemperature was ramped up to 715° C. in an atmosphere of 70% watervapor/30% N₂ followed by binder burnoff in a steam ambient.Subsequently, the atmosphere was replaced with a forming gas atmosphereand then the temperature was ramped up to 975° C. in N₂. The atmosphereis then changed to a steam ambient and heating at 975° C. continued tocomplete the second step. The pellets were then cooled down, first inthe steam ambient and then in N₂.

The shrinkage behavior of the pellets was measured during the sinteringcycle by a Netzsch dilatometer. It was observed that the glass-ceramicpellets (representing the substrate) began to shrink at about 800° C.and stopped shrinking at about 860° C., while the paste pellets began toshrink at about 800° C. and stopped shrinking at about 890° C. Thus,both sets of pellets exhibited nearly identical shrinkage behavior.According to the invention, therefore, shrinkage matching of paste andsubstrate materials is obtained.

As is now apparent, the copper-based sintering paste comprising copperaluminate proposed by the present inventors has fulfilled the dualobjectives of controlling the grain size in the sintered copper whilealso altering the shrinkage behavior of the copper particles.

It will be apparent to those skilled in the art having regard to thisdisclosure that other modifications of this invention beyond thoseembodiments specifically described here may be made without departingfrom the spirit of the invention. Accordingly, such modifications areconsidered within the scope of the invention as limited solely by theappended claims.

What is claimed is:
 1. A multilayered ceramic package comprising:aceramic substrate; and a copper-based sintering paste for formingconductive vias and surface patterns in or on said ceramic substrate,said paste comprising: powdered copper particles consisting of elementalcopper, powdered copper aluminate particles and organic materials, saidcopper aluminate constituting up to 10% by weight of said paste, whereinthe copper for said copper-based sintering paste comes exclusively fromsaid powdered copper particles and said powdered copper aluminate. 2.The ceramic package defined in claim 1 wherein said copper aluminate insaid sintering paste constitutes up to 3% by weight of said paste. 3.The ceramic package defined in claim 1 wherein said copper aluminate insaid sintering paste is present as a minimum in a small but effectiveamount to control shrinkage of said paste.
 4. The ceramic packagedefined in claim 1 wherein said copper aluminate in said sintering pasteconstitutes 0.02-10% by weight of said paste.
 5. The ceramic packagedefined in claim 4 wherein said copper aluminate constitutes 0.02-3% byweight of said paste.
 6. The ceramic package defined in claim 5 whereinsaid copper aluminate constitutes 0.02-1% by weight of said paste. 7.The ceramic package defined in claim 6 wherein said copper aluminateconstitutes 0.2-1% by weight of said paste.
 8. The ceramic packagedefined in claim 7 wherein the average size of said copper aluminateparticles is not greater than 3.0 micrometers.
 9. The ceramic packagedefined in claim 7 wherein the average size of said copper particles isabout 5-8 micrometers.
 10. The ceramic package defined in claim 1wherein in said sintering paste said copper aluminate constitutes 0.2-1%weight of said paste,the average size of said copper aluminate particlesis not greater than about 3.0 micrometers, and the average size of saidcopper particles is about 5-8 micrometers.